1. Field of the Invention
The present invention generally relates to semiconductor chip manufacture and more particularly to prevention of watermark formation on chips during manufacturing steps.
2. Background Description
A wafer drying step is crucial in the semiconductor manufacturing industry, and by reducing the formation of surface defects called "watermarks" during wafer drying, the yields can be substantially increased.
Typically after the etching of an oxide film from the silicon surface of a wafer or chip with an etchant such as a dilute hydrofluoric (DHF) or buffered hydrofluoric (BHF) solution, the wafers or chips are rinsed in deionized (DI) water to remove the chemicals. After the final rinse in DI water, the wafers are dried either in an isopropyl alcohol (IPA) dryer or a spin dryer or other suitable device. The oxide etch process makes the wafer or chip surface highly hydrophobic and reactive and this surface leaves watermark defects after drying. Park et al. conclude that spin dried hydrophobic wafers create a large number of watermarks on patterned wafers with both hydrophilic and hydrophobic sites ("Effects of Drying Methods and Wettability of Silicon on the Formation of Water Marks in Semiconductor Processing," J. Electrochem. Soc., Vol. 142, No. 6, June 1995, pp. 2028-2031).
Watermarks are always seen when wafers with a bare silicon surface (single crystal or polysilicon and H-passivation) are rinsed in high purity water in the presence of oxygen. This occurs after processes which finish with an HF-dip as the last step and all poly-chemical-mechanical polishing steps watermarks densely cover the wafer surface. While there are many steps in which watermarks are formed, each step varies as to the risk of watermark formation.
In the semiconductor industry after the final water clean steps, the wafers may be dried by spindrying, conventional type isopropyl alcohol (IPA) based drying, or other known techniques.
With respect to watermarks, Park et al. reported that spin drying results in watermarks that are around ten microns in size, and IPA drying results in watermarks of a size less than 1 micron. However, Park et al. observed watermarks with IPA dryer for patterned wafers only. The patterned wafers had hydrophilic (oxide coated and cleaned surfaces) and hydrophobic (etched surfaces) regions. All wafer dryers, except for the spin dryer, use IPA as the solvent of choice. The complete miscibility of water with IPA, and the presence of an azeotropic temperature close to 80.degree. C. may result in the formation of watermarks.